1. Field of the Invention
The present invention relates to improvements in fixing devices for image forming apparatus, such as copying machines, printers, facsimile machines, etc. Such fixing devices fix a developer or toner image printed on a paper sheet utilizing a heating element.
2. Discussion of the Background
A conventional fixing device for a copying machine, printer, facsimile or the like is composed of a heating roller which heats a toner image on a paper sheet, a pressing roller disposed in contact with the heating roller, with a halogen lamp installed in the heating roller to thereby heat the heating roller from the inside thereof. In addition, a temperature fuse is disposed outside of the heating roller, with a first sensor sensing the temperature of the center of the outer periphery of the heating roller (the center of the printing area), and a second sensor arrangement sensing the temperatures of both side edges of the outer periphery of the heating roller (the edge portions of the printing area). However, with this conventional fixing roller, the fixing is achieved without regard to the minimum amount of heat or temperature required for a corresponding paper size, resulting in a waste of electrical energy. The fixing device is particularly inefficient since the halogen lamp heats all of the heating roller from the inside thereof, regardless of the size of the paper sheet having an image fixed thereon. Furthermore, an additional sensor arrangement is required particularly where small-sized paper sheets are fixed consecutively, in order to prevent the temperature at both of the side edge portions of the heating roller from rising extraordinarily or excessively. This results in an increase in the cost of the fixing device.
Another conventional fixing device, for example as described in Japanese Laid-Open Patent Application No. 3-291681 (see, e.g., FIG. 17), is composed of a plurality of heating wires, each of which can be independently supplied with power, with each disposed inside the corresponding heating roller portions one by one, and selectively supplied with electric power to heat a toner image on the paper sheet. A few different widths of the heated part can be provided corresponding to the size of the paper sheet to be fixed. However, it is not possible to vary the heating area, or to divide the heating area into only required or desired areas or parts in the direction parallel to the axis of the heating roller, or into plural areas or parts in the circumferential direction of the heating roller. Accordingly, it is not possible to change heating widths in the direction parallel to the axis of the heating roller for various different widths, or to change the heating length in the circumferential direction of the heating roller to an extent required to more optimally utilize heat generated in the roller.